Method for forming stacked nanowire transistors

ABSTRACT

A semiconductor device includes a substrate, a first semiconductor stack including elongated semiconductor features isolated from each other and overlaid in a direction perpendicular to a top surface of the substrate, and a second semiconductor stack including elongated semiconductor features isolated from each other and overlaid in the direction perpendicular to the top surface of the substrate. The second semiconductor stack has different geometric characteristics than the first semiconductor stack. A top surface of the first semiconductor stack is coplanar with a top surface of the second semiconductor stack.

PRIORITY

This is a divisional of U.S. patent application Ser. No. 14/942,546,filed Nov. 16, 2015, herein incorporated by reference in its entirety.

BACKGROUND

In the semiconductor integrated circuit (IC) industry, technologicaladvances in IC materials and design have produced generations of ICswhere each generation has smaller and more complex circuits than theprevious generation. In the course of IC evolution, functional density(i.e., the number of interconnected devices per chip area) has generallyincreased while geometry size (i.e., the smallest component (or line)that can be created using a fabrication process) has decreased. Thisscaling down process generally provides benefits by increasingproduction efficiency and lowering associated costs. Such scaling downhas also increased the complexity of IC processing and manufacturing.

One type of transistor that helps enable such scaling down is a stackednanowire transistor. In a stacked nanowire transistor, the channel ismade of one or more elongated semiconductor features, each of which isentirely or partially surrounded by the gate structure. Such elongatedsemiconductor features may also be referred to as nanowires. Thenanowires for a single transistor may be vertically stacked.

Various transistors within an integrated circuit serve differentfunctions. For example, some transistors are designed for input/outputoperations. Some transistors are designed for core processingoperations. Some transistors are designed for memory storage operations.While it is desirable that such different transistors have differentfunctions to better serve their purposes, it can be difficult tomanufacture multiple stacked nanowire transistors in a single circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1I, and 1J are diagrams showing anillustrative process for forming stacked nanowire transistors havingvarious characteristics, according to one example of principlesdescribed herein.

FIG. 1K is a diagram showing a perspective view of a stacked nanowiretransistor, according to one example of principles described herein.

FIGS. 2A, 2B, 2C, 2D, 2E, and 2F are diagrams showing an illustrativeprocess for forming stacked nanowire transistors having variouscharacteristics, according to one example of principles describedherein.

FIGS. 3A and 3B are diagrams showing illustrative stacked nanowiretransistors having various characteristics, according to one example ofprinciples described herein.

FIG. 4 is a flowchart showing an illustrative method for forming stackednanowire transistors having various characteristics, according to oneexample of principles described herein.

FIG. 5 is a flowchart showing an illustrative method for forming stackednanowire transistors having various characteristics, according to oneexample of principles described herein.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As described above, various transistors within an integrated circuitserve different functions. While it is desirable that such differenttransistors have different functions to better serve their purposes, itcan be difficult to manufacture multiple stacked nanowire transistors ina single circuit. According to principles described herein, stackednanowire transistors may be fabricated using techniques that allow fortransistors with different characteristics. Thus, transistors can becustomized for various purposes within the integrated circuit.

FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1I, and 1J are diagrams showing anillustrative process for forming stacked nanowire transistors havingvarious characteristics. FIG. 1A illustrates a semiconductor stack 104formed onto a semiconductor substrate 102. The semiconductor stack 104includes a first plurality of semiconductor layers 106 and a secondplurality of semiconductor layers 108. The semiconductor stack 104alternates between the first plurality semiconductor layers 106 and thesecond plurality of semiconductor layers 108.

The semiconductor substrate 102 may be a semiconductor wafer used forsemiconductor fabrication processes. In one example, the semiconductorsubstrate 102 may be made of silicon. Other semiconductor materials maybe used as well. In the present example, two different regions 110, 112of the semiconductor wafer are shown. These regions 110, 112 may or maynot be adjacent to each other. As will be explained in further detailbelow, a first type of stacked nanowire transistor will be formed in thefirst region 110 and a second type of stacked nanowire transistor willbe formed in the second region 112. These two different stacked nanowiretransistors will have varying characteristics.

Each of the plurality of semiconductor layers 106, 108 may be grownthrough use of an epitaxial process. In an epitaxial process, acrystalline material is grown onto a crystalline substrate. Here, toform the first of the second plurality of semiconductor layers 108, thecrystalline substrate is the semiconductor substrate 102 and thecrystalline material to be formed on that substrate is the first ofsemiconductor layers 108. Then, to form the first of the semiconductorlayers 106, the first semiconductor layer 108 acts as the crystallinesubstrate on which the first of semiconductor layers 106 is formed.

In one example, the first plurality of semiconductor layers 106 may bemade of silicon. The second plurality of semiconductor layers 108 may bemade of silicon germanium. As will be described in further detail below,the two different materials used for the first plurality ofsemiconductor layers 106 and the second plurality of semiconductorlayers 108 are selected so that they may be selectively etched. Becausethe second plurality of semiconductor layers 108 will eventually beremoved, it is desirable to have an etching process that will remove thesecond plurality of semiconductor layers 108 while leaving the firstplurality of semiconductor layers 106 substantially intact. Othersemiconductor materials may be used as well. For example, either thefirst plurality of semiconductor layers 106 or the second plurality ofsemiconductor layers 108 may be made of one Germanium (Ge), silicongermanium (SiGe), germanium tin (GeSn), silicon germanium tin (SiGeSn),or a III-V semiconductor.

FIG. 1B illustrates a patterning process by which the semiconductorstack 104 is patterned into a plurality of semiconductor stack features114. The patterning process may be performed using various lithographictechniques. For example, a photoresist layer may be applied to the topof the semiconductor stack 104. The photoresist layer may then beexposed to a light source through a photomask. The photoresist layer maythen be developed to expose some regions of the semiconductor stack 104while covering other regions of the semiconductor stack 104. An etchingprocess may then be applied such that the exposed regions of thesemiconductor stack 104 are removed. In one example, the etching processmay be an anisotropic etching process such as a dry etching process. Theetching process may be designed to form trenches 115 to a desired depth.In the present example, the desired depth extends into the semiconductorsubstrate 102.

FIG. 1C illustrates the formation of isolation features 116 within thetrenches 115 formed by the patterning process. In some examples, theisolation features 116 may be made of a dielectric material. Theisolation features 116 may be formed by depositing the isolation featurematerial into the trenches 115 and then performing a planarizing processsuch as a chemical mechanical polishing (CMP) process to expose the topsurfaces of the semiconductor stack features 114. In some examples,before forming the isolation features 116, an oxide deposition processmay be applied to create a liner (not shown) on surfaces of thesemiconductor stack features 114 as well as the exposed portions of thesemiconductor substrate 102. An anealing process may then be applied tothe liner. In some examples, the hard mask (which may include an oxidelayer and a silicon nitride layer) used to pattern the semiconductorstack 104 may act as a CMP stop layer. Thus, after the isolationfeatures 116 are formed, there may be hard mask portions remaining overthe stack features 114. Various etching processes, such as wet etching,may be used to remove such hard mask portions.

FIG. 1D illustrates the removal of the semiconductor stack features 114within the second region 112, which leaves trenches 117 between theisolation features 116 within the second region 112. The semiconductorstack features 114 within the first region 110 remain. In one example,the semiconductor stack features 114 within the second region 112 areremoved using an etching process. The etching process may be designed toselectively remove the semiconductor stack features 114 while leavingthe isolation features 116 substantially intact. Such an etching processmay be a wet etching process or a dry etching process. To protect thesemiconductor stack features 114 within the first region 110 during sucha removal process, a photoresist layer and/or a hard mask layer (notshown) may be formed over the first region 110.

FIG. 1E is a diagram showing replacement of the semiconductor stackfeatures with a second semiconductor stack 120. Formation of the secondsemiconductor stack 120 results in semiconductor stack features 118being formed between the isolation features within the trenches 117. Thesecond semiconductor stack 120 may be formed in a manner similar to thefirst semiconductor stack 104. Specifically, the second semiconductorstack 120 may be formed using an epitaxial growth process. Like thefirst semiconductor stack 104, the second semiconductor stack 120 mayalso alternate between two different types of semiconductor materials.The second semiconductor stack 120, however, varies in characteristicsfrom the first semiconductor stack 104. In the present example, thethickness of each of the semiconductor layers within the semiconductorstack 120 is different than the thickness of semiconductor layers of thefirst semiconductor stack 104. Additionally, the number of each type oflayer in the second semiconductor stack 120 is different than the numberof each type of layer in the first semiconductor stack 104. Othervariations may be present as well. After the second semiconductor stack120 has been formed, a CMP process may be used to planarize the topsurface of the wafer so that the top surfaces of the semiconductor stackfeatures 118 are coplanar with the top surfaces of the isolationfeatures 116. Additionally, the top surfaces of the semiconductor stackfeatures 114 are essentially coplanar with the top surfaces ofsemiconductor stack features 118.

The different characteristics of the second semiconductor stack 120 canbe designed for specific types of transistors. As described above, anintegrated circuit typically includes transistors for differentfunctions. Some functions, such as input/output may benefit from athicker channel. As will be described in further detail below, one ofthe two types of semiconductor material within each of the semiconductorstack features 114, 118 will be removed. The remaining type ofsemiconductor material will be used as a channel.

FIG. 1F is a diagram showing a removal process to remove a portion ofthe isolation features 116. The isolation features 116 may be removed atportions where gate devices intended to be formed. The presentcross-section shows the region where the gate is to be formed. In thepresent example, isolation features are etched in a manner such that thetop surfaces of the isolation features 116 are coplanar with thetop-most surfaces of the semiconductor substrate 102. In some examples,top surfaces of the isolation features 116 are lower than the top-mostsurfaces of the semiconductor substrate 102.

The semiconductor stack features 114, 118 are elongated fin-likestructures that run perpendicular to the cross-section shown. In thepresent example, the first plurality of semiconductor layers 106 willform elongated semiconductor features (i.e, nanowires) that arepositioned between source and drain regions. The source and drainregions (not shown) may be formed after the removal process shown inFIG. 1F. For example, portions of the semiconductor stack features 114,118 may be removed and then replaced with a single semiconductorstructure that is doped in-situ so as to form a source or drain region.

FIGS. 1G and 1H illustrate formation of a gate device for transistorswithin the first region. FIG. 1G illustrates removal of one of the typesof semiconductor material of the first semiconductor stack features 114.Specifically, the material forming the second plurality of semiconductorlayers 108 is removed. Such material may be removed using an isotropicetching process such as a wet etching process. Removal of such materialleaves a number of elongated semiconductor features 122 suspendedbetween the source and drain regions (not shown).

In some examples, after the elongated semiconductor features 122 havebeen exposed, an additional epitaxial growth process may be applied tochange the size and/or shape of the elongated semiconductor features122. For example, it may be desired to slightly increase the widthand/or thickness of the cross-section of the elongated semiconductorfeatures 122. The epitaxial growth process may also be designed tochange the cross-sectional shape of the elongated semiconductor features122. For example, the cross-sectional shape of the elongatedsemiconductor features 122 may be rectangular, square, circular,elliptical, diamond, or other shape. In some cases, an isotropic etchingprocess may be used to reduce the size of the exposed elongatedsemiconductor features 122. Such epitaxial growth or etching processesmay be used to tune the dimensions of the elongated semiconductorfeatures 122 as desired.

FIG. 1H illustrates formation of a gate structure 124 within the firstregion 110. In the present example, the gate structure 124 wraps aroundeach side of the elongated semiconductor features 122. The gatestructure 124 also electrically connects the gate devices for a numberof stacked nanowire transistors 123 formed within the first region 110.

In some examples, the elongated semiconductor features 122 may undergovarious treatment and cleaning processes before the gate structure 124is formed. For example, a thermal treatment may be applied to theelongated semiconductor features 122 with a temperature within a rangeof about 650-1000 degrees Celsius. A cleaning process may be used toremove any native oxygen.

The gate structure 124 may include a number of materials. In someexamples, the gate structure may include an interfacial layer (notshown), a high-k dielectric layer (not shown), and a metal gate layer.The interfacial layer may be formed first. The interfacial layer maywrap around and contact each side of each of the elongated semiconductormaterials 122. The interfacial layer may include an oxide-containingmaterial such as silicon oxide or silicon oxynitride, and may be formedby chemical oxidation using an oxidizing agent (e.g., hydrogen peroxide(H₂O₂), ozone (O₃)), plasma enhanced atomic layer deposition, thermaloxidation, ALD, CVD, and/or other suitable methods.

After the interfacial layer is formed, a high-k dielectric layer may beformed around each of the elongated semiconductor features 122 over theinterfacial layer. The high-k dielectric material has a high dielectricconstant, for example, greater than that of thermal silicon oxide(˜3.9). The high-k dielectric material may include hafnium oxide (HfO₂),zirconium oxide(ZrO₂), lanthanum oxide(La₂O₃), aluminum oxide (Al₂O₃),titanium oxide (TiO₂), yttrium oxide, strontium titanate, hafniumoxynitride (HfO_(x)N_(y)), other suitable metal-oxides, or combinationsthereof. The high-k dielectric layer may be formed by ALD, chemicalvapor deposition (CVD), physical vapor deposition (PVD), remote plasmaCVD (RPCVD), plasma enhanced CVD (PECVD), metal organic CVD (MOCVD),sputtering, other suitable processes, or combinations thereof.

After the interfacial layer and high-k dielectric layer are formed, thegate layer may be formed. The gate layer includes a conductive materialsuch as a metal material. For example, the gate layer may includetungsten, titanium, tantalum, or other suitable metal gate material. Thegate layer may be formed using a variety of suitable depositionprocesses. In the present example, the gate layer also interconnectsmultiple transistors (formed by the multiple elongated structure stacks)shown within the first region 110. The present example illustrates agate-all-around (GAA) structure in which the gate is wrapped all aroundthe nanowire structure. In some examples, however, the gate structuremay wrap partially around the nanowire structure.

FIGS. 1I and 1J illustrate formation of gate devices for transistorswithin the second region 112. FIG. 1I illustrates removal of one of thetypes of semiconductor material of the second semiconductor stackfeatures 118. Specifically, the material forming the second plurality ofsemiconductor layers 108 is removed. Such material may be removed usingan isotropic etching process such as a wet etching process. Removal ofsuch material leaves a number of elongated semiconductor features 126suspended between the source and drain regions (not shown).

In some examples, after the elongated semiconductor features 126 havebeen exposed, an additional epitaxial growth process may be applied tochange the size and/or shape of the elongated semiconductor features126. For example, it may be desired to slightly increase the widthand/or thickness of the cross-section of the elongated semiconductorfeatures 126. In some cases, an isotropic etching process may be used toreduce the size of the exposed elongated semiconductor features 126.Such epitaxial growth or etching processes may be used to tune thedimensions of the elongated semiconductor features 126 as desired. Forexample, the cross-sectional shape of the elongated semiconductorfeatures 126 may be rectangular, square, circular, elliptical, diamond,or other shape. The size and shape of the elongated semiconductorfeatures 126 may be different than the size and shape of the elongatedsemiconductor features 122.

FIG. 1J illustrates formation of a gate structure 128 within the secondregion 112. In the present example, the gate structure 128 wraps aroundeach side of the elongated semiconductor features 126. The gatestructure 128 also electrically connects the gate devices for a numberof stacked nanowire transistors 125 formed within the second region 112.

In some examples, the elongated semiconductor features 126 may alsoundergo various treatment and cleaning processes before the gatestructure 128 is formed. The gate structure 128 may also include anumber of materials. For example, like the gate structure 124, gatestructure 128 may include an interfacial layer, a high-k dielectriclayer, and a metal gate layer. In some examples, the thicknesses of theinterfacial layer and the high-k dielectric layer for the gate structure128 may be different than the thicknesses of the interfacial layer andthe high-k dielectric layer for the gate structure 124. The metalmaterial used for the gate structure 128 may be different than the metalmaterial used for gate structure 124.

While the stacked nanowire transistors 123, 125 have varyingcharacteristics, such as different thicknesses, different pitches, anddifferent number of nanowires, the top surfaces of the top-mostelongated semiconductor features 122, 126 from both stacked nanowiretransistors 123,125 are substantially coplanar. Thus, despite differentdevice characteristics, regions 110 and 112 of the wafer aresubstantially planar. This simplifies formation of subsequent layers.For example, an interlayer dielectric layer (ILD) may be formed on topof the stacked nanowire transistors 123, 125. Various interconnects maythen be formed within the ILD layer. In some examples, the bottomsurfaces of the bottom-most elongated semiconductor features 122, 126may be substantially coplanar. In some examples, however, the bottomsurfaces of the bottom-most elongated semiconductor features 122, 126may be offset from each other.

FIG. 1K is a diagram showing a perspective view of a stacked nanowiretransistor 150 that includes a stack of elongated semiconductor features151. The stacked nanowire transistor 150 may correspond to one of thestacked nanowire transistors 123, 125 shown in FIG. 1J. The elongatedsemiconductor features 151 may correspond to the elongated semiconductorfeatures 122, 126 shown in FIG. 1J. According to the present example,the elongated semiconductor features 151 are shown stacked on top ofeach other. The stacked nanowire transistor 150 includes a firstsource/drain region 152, a first spacer 154, a gate region 156, a secondspacer 158, and a second source/drain region 160. The first spacer 154is positioned between the first source/drain region 152 and the gateregion 156. The second spacer 158 is positioned between the gate region156 and the second source/drain region 160. FIGS. 1A-1J illustrate across-section through the gate region 156 as the stacked nanowiretransistor 150 is formed.

The portions of the elongated semiconductor features 151 that passthrough the gate region 156 function as a channel for the stackednanowire transistor 150. The portions of the elongated semiconductorfeatures 151 that pass through the source/drain regions 152, 160function as a source and drain for the stacked nanowire transistor 150.The source/drain regions 152/160 may be electrically connected tosource/drain contacts (not shown). Similarly, the gate region 156 may beelectrically connected to a gate contact (not shown). Thus, the stackednanowire transistor 150 is able to function within the integratedcircuit.

FIGS. 2A, 2B, 2C, 2D, 2E, and 2F are diagrams showing an illustrativeprocess for forming stacked nanowire transistors having variouscharacteristics. FIGS. 2A-2F illustrate a process in which the secondsemiconductor stack is formed before both semiconductor stacks arepatterned. FIG. 2A illustrates a first semiconductor stack 206 formedonto a semiconductor substrate 102. The semiconductor stack 206 includesa first plurality of semiconductor layers 208 and a second plurality ofsemiconductor layers 210. The semiconductor stack 206 alternates betweenthe first plurality of semiconductor layers 208 and the second pluralityof semiconductor layers 210.

In the present example, two different regions 202, 204 of thesemiconductor substrate 102 are shown. These regions 202, 204 may or maynot be adjacent to each other. As will be explained in further detailbelow, a first type of stacked nanowire transistor will be formed in thefirst region 202 and a second type of stacked nanowire transistor willbe formed in the second region 204. These two different devices willhave varying characteristics.

Each of the plurality of semiconductor layers 208, 210 may be grownthrough use of an epitaxial growth process. In one example, the firstplurality of semiconductor layers 208 may be made of silicon. The secondplurality of semiconductor layers 210 may be made of silicon germanium.As will be described in further detail below, the two differentmaterials used for the first plurality of semiconductor layers 208 andthe second plurality of semiconductor layers 210 are selected so thatthey may be selectively etched. Because the second plurality ofsemiconductor layers 210 will eventually be removed, it is desirable tohave an etching process that will remove the second plurality ofsemiconductor layers 210 while leaving the first plurality ofsemiconductor layers 208 substantially intact. Other semiconductormaterials may be used. For example, either the first plurality ofsemiconductor layers 208 or the second plurality of semiconductor layers210 may be made of one silicon germanium, germanium tin (GeSn), silicongermanium tin (SiGeSn), or a III-V semiconductor.

According to the present example, a patterned mask 212 is used toprotect some regions of the semiconductor stack 206 while exposing otherregions of the semiconductor stack 206. Specifically, the regionsintended to be replaced are exposed and the regions intended to remainare covered by the patterned mask 212. In the present example, thepatterned mask 212 protects the semiconductor stack 206 over the firstregion 202 while exposing the semiconductor stack 206 over the secondregion 204.

FIG. 2B is a diagram showing removal of the exposed regions of thesemiconductor stack 206. The exposed regions, i.e., region 204, may beremoved using an anisotropic etching process such as a dry etchingprocess. During such a process, the patterned mask 212 protects thesemiconductor stack 206 over the first region 202.

FIG. 2C is a diagram showing an illustrative formation process to form asecond semiconductor stack 214 within the second region 204. The secondsemiconductor stack 214 alternates between a first plurality ofsemiconductor layers 216 and a second plurality of semiconductor layers218. The second semiconductor stack 214 is similar to the firstsemiconductor stack 206 but has varying characteristics. For example,the second semiconductor stack 214 may have different semiconductormaterials than the first semiconductor stack 206. Additionally, thesecond semiconductor stack 214 may have a different number of layersthan the first semiconductor stack 206. The layers within the secondsemiconductor stack 214 may have different thicknesses and pitches thanthe layers of the first semiconductor stack 206. The secondsemiconductor stack may be formed using an epitaxial growth process.After the second semiconductor stack 214 has been formed, a CMP processmay be used to planarize the top surface of the wafer.

FIG. 2D illustrates a patterning process to form a first set ofsemiconductor stack features 220 within the first region 202 and asecond set of semiconductor stack features 222 within the second region204. Such patterning may be similar to the patterning described above inaccordance with the text accompanying FIG. 1B. The patterning may resultin fin structures within the semiconductor substrate 102.

FIG. 2E is a diagram showing formation of isolation regions 221 betweenthe semiconductor stack features 220, 222. The isolation features 221may be formed by depositing a dielectric material within the spaces (ordistances) between the semiconductor stack features 220, 222. Then, anetching process may be used to tune the height of the isolation featuresso that they are substantially coplanar with the top-most surfaceswithin the semiconductor substrate 102. In some examples, top surfacesof the isolation features 221 are lower than the top-most surfaces ofthe semiconductor substrate 102. The isolation features 221 may beformed in a manner similar to the isolation features described above inaccordance with the text accompanying FIGS. 1E-1F.

FIG. 2F is a diagram showing a first set of stacked nanowire transistors223 within the first region 202 and a second set of stacked nanowiretransistors 225 within the second region 204. The stacked nanowiretransistors 223, 225 may be formed similar to the stacked nanowirefeatures described above in the text accompanying FIGS. 1G-1J.Specifically, for the first region 202, the second plurality ofsemiconductor layers 210 are removed from the semiconductor stackfeatures 220. Then, a gate device 224 is formed around each of theremaining elongated semiconductor features 227 of the stacked nanowiretransistors 223. For the second region 204, one type of semiconductormaterial is removed from the semiconductor stack features 222. Then, agate device 226 is formed around each of the remaining elongatedsemiconductor features 229 of the stacked nanowire transistors 225.

While FIGS. 2A-2F illustrate a process by which two different types ofstacked nanowire transistors are formed, other processes usingprinciples described herein may be used to form more than two types ofstacked nanowire transistors. For example, a portion of the firstsemiconductor stack may be removed from a third region. Then, a thirdsemiconductor stack may be formed within the third region. The thirdsemiconductor stack may have features that vary from both the firstsemiconductor stack 206 and the second semiconductor stack 214.

FIGS. 3A and 3B are diagrams showing illustrative stacked nanowiretransistors with various characteristics. FIG. 3A illustrates a firsttype of stacked nanowire transistor 301 and a second type of stackednanowire transistor 303. Each of the first type of stacked nanowiretransistors 301 has four elongated semiconductor features 307 that arevertically stacked. Each of the second type of stacked nanowiretransistors 303 also has four elongated semiconductor features 309 thatare vertically stacked. Thus, in the present example, both types ofstacked nanowire transistors 301, 303 have the same number of elongatedsemiconductor features in each transistor. Additionally, both theelongated semiconductor features 307 and the elongated semiconductorfeatures 309 are made of the same semiconductor material.

In the present example, the thickness 308 of the elongated semiconductorfeatures 309 is smaller than the thickness 304 of the elongatedsemiconductor features 307. Additionally, the space (or distance) 310between the elongated semiconductor features 309 is larger than thespace (or distance) 306 between the elongated semiconductor features307. Consequently, the pitch 322 between the elongated semiconductorfeatures 309 is different than the pitch 320 between the elongatedsemiconductor features 307. In some examples, as is the case for stackednanowire transistor 301, the space (or distance) 306 between elongatedsemiconductor features 307 is equal to the thickness 304 of theelongated semiconductor features 307. However, the space (or distance)310 between elongated semiconductor features 309 is different than thethickness 308 of elongated semiconductor features 309. In the presentexample, the space (or distance) 310 is larger than the thickness 308.In some examples, however, the space (or distance) between elongatedsemiconductor features may be less than the thickness of the elongatedsemiconductor features. The thickness of the elongated semiconductorfeatures 307, 309 may be within a range of about 3-20 nanometers.Furthermore, in the present example, the top surfaces of the top-mostelongated semiconductor feature 307, 309 of both types of stackednanowire transistor 301, 303 are substantially coplanar along plane 302.

FIG. 3B illustrates a first type of stacked nanowire transistor 301 anda third type of stacked nanowire transistor 305. While the first type ofstacked nanowire transistor 301 has four elongated semiconductorfeatures 307, the third type of stacked nanowire transistor 305 has onlytwo elongated semiconductor features 311 that are vertically stacked.Thus, first type of stacked nanowire transistor 301 has a differentnumber of elongated semiconductor features than the third type ofstacked nanowire transistor 305. Additionally, the elongatedsemiconductor features 311 are made of a different semiconductormaterial than the elongated semiconductor features 307.

In the present example, the thickness 312 of the elongated semiconductorfeature 311 is greater than the thickness 304 of the longestsemiconductor features 307. Additionally, the space (or distance) 314between the elongated semiconductor features 211 is greater than thespace (or distance) 306 between the elongated semiconductor features307. Consequently, the pitch 324 between the elongated semiconductorfeatures 311 is different than the pitch 320 between the elongatedsemiconductor features 307. Furthermore, the top surfaces of thetop-most elongated semiconductor feature 307, 311 of both types ofstacked nanowire transistor 301, 305 are substantially coplanar alongplane 302.

FIG. 4 is a flowchart showing an illustrative method 400 for formingstacked nanowire transistors having various characteristics and in whichthe semiconductor stack for a second type of stacked nanowire transistoris formed after the semiconductor stack for the first type of stackednanowire transistor is patterned. According to the present example, themethod 400 includes a step 402 for forming a first semiconductor stackusing an epitaxial growth process. The first semiconductor stackincludes a first plurality of semiconductor layers alternating with asecond plurality of semiconductor layers. The first plurality ofsemiconductor layers includes a first semiconductor material and thesecond plurality of semiconductor layers includes a second semiconductormaterial that is different than the first semiconductor material. Boththe first plurality of semiconductor layers and the second pluralitysemiconductor layers may be formed as described above in the textaccompanying FIG. 1A.

According to the present example, the method 400 further includes a step404 for patterning the first semiconductor stack to form a set ofsemiconductor stack features. The set of semiconductor stack featuresmay include features that will ultimately become a first type of stackednanowire transistor and features that will become a second type ofstacked nanowire transistor. The patterning process may be performed asdescribed above in accordance with the text accompanying FIG. 1B.

According to the present example, the method 400 further includes a step406 for forming isolation features between the semiconductor stackfeatures. The isolation features may be formed in a first regioncorresponding to the first type of stacked nanowire transistor and asecond region corresponding to the second type of stacked nanowiretransistor. The isolation features may be formed as described above inthe text accompanying FIG. 1C.

According to the present example, the method 400 further includes a step408 for removing at least one of the semiconductor stack features,thereby forming at least one trench. For example, one of thesemiconductor stack features within the region corresponding to thesecond type of stacked nanowire transistor is removed. Such a removalprocess may be performed as described above in the text accompanyingFIG. 1D.

According to the present example, the method 400 further includes a step410 for forming, within the trench, a second semiconductor stack usingan epitaxial growth process, the second semiconductor stack havingdifferent characteristics than the first semiconductor stack. The secondsemiconductor stack will ultimately become a second type of stackednanowire transistor. Forming the second semiconductor stack may beperformed as described above the text accompanying FIG. 1E. Both thefirst type of stacked nanowire transistor and the second type of stackednanowire transistor may be completed as described above in the textaccompanying FIGS. 1F-1J.

FIG. 5 is a flowchart showing an illustrative method for forming stackednanowire transistors having various characteristics and in which thesemiconductor stacks for both a first type of stacked nanowiretransistor and a second type of stacked nanowire transistor are formedbefore the semiconductor stacks for both types of stacked nanowiretransistors are patterned. According to the present example, the method500 includes a step 502 for forming, on a substrate, a firstsemiconductor stack. The first semiconductor stack includes a firstplurality of semiconductor layers alternating with a second plurality ofsemiconductor layers, the first plurality of semiconductor layersincludes a first semiconductor material and the second plurality ofsemiconductor layers includes a second semiconductor material that isdifferent than the first semiconductor material. The first semiconductorstack may be formed as described above in the text accompanying FIG. 2A.

According to the present example, the method 500 further includes a step504 for removing a first portion of the first semiconductor stack over afirst region of the substrate while leaving a second portion of thefirst semiconductor stack over a second region of the substrate. Thismay be done using various photolithographic patterning techniques. Forexample, this may be done as described above in the text accompanyingFIG. 2B. In this case, the first region corresponds to region 204 andthe second region corresponds to region 202.

According to the present example, the method 500 further includes a step506 for forming, on the first region of the substrate, a secondsemiconductor stack, the second semiconductor stack having differentcharacteristics than the first semiconductor stack. Second semiconductorstack formed in a manner similar to that of the first semiconductorstack. The second semiconductor stack may be formed as described abovethe text accompanying FIG. 2C.

According to the present example, the method 500 further includes a step508 for patterning the first semiconductor stack and the secondsemiconductor stack to form a first set of semiconductor stack featuresover the first region and a second set of semiconductor stack featuresover the second region. This patterning process may be formed asdescribed above in the text accompanying FIG. 2D. In this case, thefirst set of semiconductor stack features correspond to features 222 andthe second set of semiconductor stack features correspond to features220. The stacked nanowire transistors may then be completed as describedin the FIGS. 2E-2F.

The methods and processes described herein may be used in accordancewith methods to form finFET and planar transistors. For example, sometypes of transistors within the circuit may include the stacked nanowiretransistors as described above and some transistors within theintegrated circuit may be finFET transistors or planar transistors. Inone example, core transistors are different types of stacked nanowiretransistors as described above and input/output transistors are finFETor planar transistors. Other combinations are contemplated as well.

Using principles described herein, various types of stacked nanowiretransistors may be formed using an efficient process flow. Specifically,such stacked nanowire transistors may have varying characteristicssuited for different transistor functions such as input/output, storage,and core transistors. The different types of stacked nanowiretransistors can be formed using the processes described above to havevarying characteristics of the stacked nanowires (elongatedsemiconductor structures). Additionally, despite having various nanowirecharacteristics, the top-most nanowires from each of the varying stackednanowire transistors may be substantially coplanar.

According to one example, a method includes forming a firstsemiconductor stack using an epitaxial growth process, the firstsemiconductor stack comprising a first plurality of semiconductor layersalternating with a second plurality of semiconductor layers, the firstplurality of semiconductor layers comprising a first semiconductormaterial and the second plurality of semiconductor layers comprising asecond semiconductor material that is different than the firstsemiconductor material. The method further includes patterning the firstsemiconductor stack to form a set of semiconductor stack features,forming isolation features between the semiconductor stack features,removing at least one of the semiconductor stack features, therebyforming at least one trench, and forming, within the trench, a secondsemiconductor stack using an epitaxial growth process, the secondsemiconductor stack having different characteristics than the firstsemiconductor stack.

According to one example, a method including forming, on a substrate, afirst semiconductor stack, the first semiconductor stack comprising afirst plurality of semiconductor layers alternating with a secondplurality of semiconductor layers, the first plurality of semiconductorlayers comprising a first semiconductor material and the secondplurality of semiconductor layers comprising a second semiconductormaterial that is different than the first semiconductor material. Themethod further includes removing a first portion of the firstsemiconductor stack over a first region of the substrate while leaving asecond portion of the first semiconductor stack over a second region ofthe substrate, forming, on the first region of the substrate, a secondsemiconductor stack, the second semiconductor stack having differentcharacteristics than the first semiconductor stack, and patterning thefirst semiconductor stack and the second semiconductor stack to form afirst set of semiconductor stack features over the first region and asecond set of semiconductor stack features over the second region.

According to one example, a semiconductor device includes a firststacked elongated semiconductor feature transistor having a first set ofelongated semiconductor features isolated from each other and arrangedalong a line in a direction perpendicular to the substrate, the firstset of elongated features comprising a first set of characteristics. Thesemiconductor device further includes a second stacked elongatedsemiconductor feature transistor having a second set of elongatedsemiconductor features isolated from each other and arranged along aline in a direction perpendicular to the substrate, the second set ofelongated features comprising a second set of characteristics that isdifferent than the first set of characteristics.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a first semiconductor stack including elongated semiconductor featuresspaced apart from each other and overlaid in a direction perpendicularto a top surface of the substrate; and a second semiconductor stackincluding elongated semiconductor features spaced apart from each otherand overlaid in the direction perpendicular to the top surface of thesubstrate, the second semiconductor stack having different geometriccharacteristics than the first semiconductor stack, a top surface of atopmost elongated semiconductor feature of the first semiconductor stackbeing coplanar with a top surface of a topmost elongated semiconductorfeature of the second semiconductor stack, wherein the topmost elongatedsemiconductor feature of the first semiconductor stack is formed of adifferent material than the topmost elongated semiconductor feature ofthe second semiconductor stack, wherein a bottommost elongatedsemiconductor feature from the first semiconductor stack is closer tothe substrate than a bottommost elongated semiconductor feature from thesecond semiconductor stack without either bottommost elongatedsemiconductor features from the first or second semiconductor stacksphysically directly contacting the substrate.
 2. The semiconductordevice of claim 1, further comprising: an isolation feature disposed inthe substrate adjacent the first semiconductor stack.
 3. Thesemiconductor device of claim 1, wherein the first semiconductor stackhas a different pitch than the second semiconductor stack.
 4. Thesemiconductor device of claim 1, wherein the elongated semiconductorfeatures of the first semiconductor stack have different thicknessesthan the elongated semiconductor features of the second semiconductorstack.
 5. The semiconductor device of claim 1, wherein: the firstsemiconductor stack has a smaller pitch than the second semiconductorstack; and the elongated semiconductor features of the firstsemiconductor stack have smaller thicknesses than the elongatedsemiconductor features of the second semiconductor stack.
 6. Thesemiconductor device of claim 1, wherein a number of the elongatedsemiconductor features of the first semiconductor stack is differentthan a number of the elongated semiconductor features of the secondsemiconductor stack.
 7. The semiconductor device of claim 6, wherein thenumber of the elongated semiconductor features of the firstsemiconductor stack is twice of the number of the elongatedsemiconductor features of the second semiconductor stack.
 8. Thesemiconductor device of claim 1, wherein the elongated semiconductorfeatures of the first semiconductor stack have different cross-sectionalshapes than the elongated semiconductor features of the secondsemiconductor stack.
 9. The semiconductor device of claim 8, whereincross-sectional shapes of the elongated semiconductor features of thefirst and second semiconductor stacks are selected from rectangular,square, circular, elliptical, and diamond.
 10. The semiconductor deviceof claim 1, wherein the elongated semiconductor features of the firstsemiconductor stack are also different from the elongated semiconductorfeatures of the second semiconductor stack in material compositions. 11.A semiconductor device, comprising: a substrate; a first nanowire stackincluding a first plurality of nanowires; and a second nanowire stackincluding a second plurality of nanowires, wherein a nanowire of thefirst plurality of nanowires has a different thickness than a nanowireof the second plurality of nanowires, and wherein a top surface of atop-most nanowire of the first plurality of nanowires is coplanar with atop surface of a top-most nanowire of the second plurality of nanowires,wherein a bottommost nanowire from the first nanowire stack is closer tothe substrate than a bottommost nanowire from the second nanowire stackwithout either bottommost nanowires from the first or second nanowirestacks directly contacting the substrate, wherein the bottommostnanowire from the second nanowire stack is thicker than the bottommostnanowire from the first nanowire stack, wherein the top-most nanowire ofthe first plurality of nanowires is formed of a different material thanthe top-most nanowire of the second plurality of nanowires.
 12. Thesemiconductor device of claim 11, wherein the first plurality ofnanowires has a different pitch than the second plurality of nanowires.13. The semiconductor device of claim 11, wherein the first plurality ofnanowires includes more elongated, vertically stacked features than thesecond plurality of nanowires.
 14. The semiconductor device of claim 11,wherein the first plurality of nanowires has a number of nanowiresdifferent than that of the second plurality of nanowires.
 15. Asemiconductor device, comprising: a substrate; a first nanowire stackincluding a first plurality of nanowires; a second nanowire stackincluding a second plurality of nanowires, the second nanowire stackhaving different geometric characteristics than the first nanowirestack; and an isolation feature disposed in the substrate adjacent thefirst nanowire stack, and wherein a bottommost nanowire from the firstplurality of nanowires is closer to the substrate than a bottommostnanowire from the second plurality of nanowires without eitherbottommost nanowires from the first or second plurality of nanowiresdirectly contacting the substrate, wherein the bottommost nanowire fromthe second plurality of nanowires is thicker than the bottommostnanowire from the first plurality of nanowires, wherein the firstplurality of nanowires has a different number of nanowires than thesecond plurality of nanowires.
 16. The semiconductor device of claim 15,wherein the first plurality of nanowires is different from the secondplurality of nanowires in at least one of: pitch, nanowire thickness, orshape of nanowires.
 17. The semiconductor device of claim 15, wherein atop surface of a topmost nanowire of the first nanowire stack iscoplanar with a top surface of a topmost nanowire of the second nanowirestack.
 18. The semiconductor device of claim 15, wherein the firstplurality of nanowires is formed from a material selected from the groupconsisting of a Si and SiGe, and wherein the second plurality ofnanowires is formed from a material selected from the group consistingof a Si and SiGe.
 19. The semiconductor of device 15, wherein eachelongated semiconductor feature of the first semiconductor stack has afirst thickness, wherein the first semiconductor stack is part of afirst transistor in a single circuit, and wherein each elongatedsemiconductor feature of the second semiconductor stack has a secondthickness that is greater than the first thickness, wherein the secondsemiconductor stack is part of a second transistor in the singlecircuit, the second transistor performing a different function than thefirst transistor, wherein an input/output operation of the secondtransistor improves because of the thicker elongated semiconductorfeatures of the second semiconductor stack.